Printed circuit boards were nearly unkown only a little more than a score of years ago. Now they are reqularly used in such diverse products as radios and ranges; watches and washing machines; computers and clocks or TV sets and telephones. Printed circuit boards come in a wide variety of shapes, sizes and configurations. In many products, such as computers, communication systems, industrial process control systems and/or many other applications, it is frequently necessary to provide several dozen printed circuit boards, some of which may be indentical to others, while others may vary in both size and circuit configuration. For efficient space utilization, it is generally desirable to package as many printed circuit boards, and other components of the system, in a minimum volume. This creates a variety of ancillary problems, including the fact that it may be difficult or impossible to see into certain areas to inspect for fault conditions. The close proximity of the printed circuit boards and other components introduces two other potential problems which are solved by the present invention. One of these problems relates to the heat that is generated when electrical components are densely packed, thereby increasing the possibility for heat to build up and cause damage. This possibility is increased if the density and packaging configuration limits air circulation. The other problem that arises with densely packed components is that there may be inadvertent physical contact between elements which should not be in contact and that such inadvertent contact may introduce errors or cause serious damage to the equipment. The probability of inadvertent contact is aggravated when the clearances are small and there is any possibility of movement of one element relative to another.
Typically, printed circuit boards are substantially planar with small elements or components mounted thereon. The conductors on the printed circuit board are usually arranged in a group at a projection near one edge of the board and the projection plugs into a connector. Wiring between the plurality of connectors, and other components, provides the necessary interconnections between the printed circuit boards and the components thereon. The most common sizes for printed circuit boards of the type presently under discussion range from approximately the size of a small index card to approximately the size of a sheet of typing paper. Depending upon the size and weight of a printed circuit board and its orientation and proximity to other cards and components, the card may be supported solely by the projection that plugs into the connector. If other support is not provided, it is easy to visualize how the printed circuit boards may move if the apparatus is subjected to a blow or vibration. Any such movement might result in the aforementioned inadvertent contact between elements on adjacent printed circuit boards and/or other elements mounted nearby. An obvious solution to this problem is to provide edge supports and guides to eliminate, or at least minimize, the extent of the motion which may take place. This solution is entirely practical and many types of edge supports have been provided. However, it should be immediately apparent that the use of edge supports obviously adds to the cost, weight and size of the total structure. In addition, the use of edge supports aggravates the potential danger of overheating for at least the reason that the edge supports tend to limit air circulation.
Various solutions have been suggested to minimize the overheating problems while still providing adequate support. These solutions include mounting the printed circuit boards so that their planes are substantially vertical, rather than horizontal, so that the possibility of cooling by convection air currents is improved. In addition, blowers have been employed to provide forced air cooling. Also, a wide variety of edge supports have been provided to try to provide the required support while still allowing as much air circulation as possible.
One typical support method may be compared with the techniques used for supporting racks in a domestic oven where the printed circuit boards may be compared with the oven racks. The card supports in this analogy may be compared with the oven walls and may be slotted or perforated to permit some air to circulate. Another widely used system comprises a U-shaped channel which supports an edge of the printed curcuit card and which may be supported at each end by some means which provides as much support as is practical without totally inhibiting all air circulation.